Planetary gear train of automatic transmission for vehicle

ABSTRACT

A planetary gear train of an automatic transmission for a vehicle may include an input shaft receiving torque of an engine, an output shaft outputting torque of the engine, a first planetary gear set, a second planetary gear set, a third planetary gear set, a fourth planetary gear set, a first shaft, a second shaft directly connected to the input shaft, a third shaft, a fourth shaft, a fifth shaft selectively connectable to the third shaft, a sixth shaft selectively connectable to the third shaft, a seventh shaft selectively connectable to the fourth shaft, and an eighth shaft selectively connectable to the third shaft, and directly connected to the output shaft.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to Korean Patent Application No.10-2016-0032328, filed Mar. 17, 2016, the entire contents of which isincorporated herein for all purposes by this reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an automatic transmission for avehicle. More particularly, the present invention relates to a planetarygear train of an automatic transmission for a vehicle that improvespower delivery performance and fuel economy by achieving nine forwardspeed stages with a minimum number of constituent elements being usedand improves silent driving of the vehicle by using an operation pointpositioned at a low engine speed.

Description of Related Art

Generally, an automatic transmission achieving more speed stages hasbeen developed to enhance fuel economy and optimize drivability. Recentincreases in oil prices are triggering stiff competition in enhancingfuel consumption of a vehicle.

Therefore, much research for reducing weight and enhancing fuel economythrough downsizing of an engine and for securing drivability and fueleconomy through multiple speed stages of automatic transmissions hasbeen developed.

However, in the automatic transmission, as the number of speed stagesincrease, the number of internal components (particularly, planetarygear sets) increase, and as a result, a length of the transmissionincreases. This may deteriorate installability and/or power flowefficiency and may increase production cost, and weight.

Accordingly, development of a planetary gear train which may achievemaximum efficiency with a small number of components may be important inorder to increase a fuel economy enhancement effect through the multiplespeed-stages.

In this aspect, in recent years, 8-speed automatic transmissions tend tobe implemented and the research and development of a planetary geartrain capable of implementing more speed stages has also been activelyconducted.

However, a conventional 8-speed automatic transmission typicallyincludes three to four planetary gear sets and five to six controlelements (frictional elements). In this case, since the length of theautomatic transmission increases, mountability may be deteriorated.

In recent planetary gear trains, one planetary gear set is disposedabove another planetary gear set, but structures of automatictransmissions to which parallel planetary gear sets is applied are verylimited.

In other planetary gear trains, dog clutches instead of control elementsof wet-type are used. However, shift feel may be deteriorated.

The information disclosed in this Background of the Invention section isonly for enhancement of understanding of the general background of theinvention and should not be taken as an acknowledgement or any form ofsuggestion that this information forms the prior art already known to aperson skilled in the art.

BRIEF SUMMARY

Various aspects of the present invention are directed to providing aplanetary gear train of an automatic transmission for a vehicle havingadvantages of improving power delivery performance and fuel economy byachieving at least nine forward speed stages and one reverse speedstage.

In addition, various aspects of the present invention are directed toproviding a planetary gear train of an automatic transmission for avehicle having further advantages of improving silent driving of thevehicle by using operation point positioned at a low rotation speedregion of an engine.

According to various aspects of the present invention, a planetary geartrain of an automatic transmission for a vehicle may include an inputshaft receiving torque of an engine, an output shaft outputting torqueof the engine, a first planetary gear set including first, second, andthird rotation elements, a second planetary gear set including fourth,fifth, and sixth rotation elements, a third planetary gear set includingseventh, eighth, and ninth rotation elements, a fourth planetary gearset including tenth, eleventh, and twelfth rotation elements, a firstshaft connecting the first rotation element to the fifth rotationelement and the ninth rotation element, a second shaft connected to thesecond rotation element and directly connected to the input shaft, athird shaft connecting the third rotation element to the seventhrotation element, a fourth shaft connected to the fourth rotationelement, a fifth shaft connected to the sixth rotation element andselectively connectable to the third shaft, a sixth shaft connecting theeighth rotation element to the twelfth rotation element and selectivelyconnectable to the third shaft, a seventh shaft connected to the tenthrotation element and selectively connectable to the fourth shaft, and aneighth shaft connected to the eleventh rotation element, selectivelyconnectable to the third shaft, and directly connected to the outputshaft.

The first shaft and the fourth shaft may each be selectively connectableto a transmission housing.

The first, second, and third rotation elements may be a first sun gear,a first planet carrier, and a first ring gear, the fourth, fifth, andsixth rotation elements may be a second sun gear, a second planetcarrier, and a second ring gear, the seventh, eighth, and ninth rotationelements may be a third sun gear, a third planet carrier, and a thirdring gear, and the tenth, eleventh, and twelfth rotation elements may bea fourth sun gear, a fourth planet carrier, and a fourth ring gear.

The planetary gear train may further include a first clutch selectivelyconnecting the third shaft to the eighth shaft, a second clutchselectively connecting the third shaft to the fifth shaft, a thirdclutch selectively connecting the third shaft to the sixth shaft, afourth clutch selectively connecting the fourth shaft to the seventhshaft, a first brake selectively connecting the first shaft to thetransmission housing, and a second brake selectively connecting thefourth shaft to the transmission housing.

A planetary gear train of an automatic transmission for a vehicle mayinclude an input shaft receiving torque of an engine, an output shaftoutputting torque, a first planetary gear set including first, second,and third rotation elements, a second planetary gear set includingfourth, fifth, and sixth rotation elements, a third planetary gear setincluding seventh, eighth, and ninth rotation elements, and a fourthplanetary gear set including tenth, eleventh, and twelfth rotationelements, in which the input shaft may be directly connected to thesecond rotation element, the output shaft may be directly connected tothe eleventh rotation element, the first rotation element may bedirectly connected to the fifth rotation element and the ninth rotationelement, the third rotation element may be directly connected to theseventh rotation element, the sixth rotation element may be selectivelyconnectable to the third rotation element, the eighth rotation elementmay be directly connected to the twelfth rotation element andselectively connectable to the seventh rotation element, the tenthrotation element may be selectively connectable to the fourth rotationelement, and the eleventh rotation element may be selectivelyconnectable to the third rotation element.

The first rotation element and the fourth rotation element may each beselectively connectable to a transmission housing.

The planetary gear train may further include a first clutch selectivelyconnecting the seventh rotation element to the eleventh rotationelement, a second clutch selectively connecting the third rotationelement to the sixth rotation element, a third clutch selectivelyconnecting the seventh rotation element to the eighth rotation element,a fourth clutch selectively connecting the fourth rotation element tothe tenth rotation element, a first brake selectively connecting thefirst rotation element to the transmission housing, and a second brakeselectively connecting the fourth rotation element to the transmissionhousing.

Speed stages implemented by selectively operating the first, second,third and fourth clutches and the first and second brakes include aforward first speed stage, implemented by operating the first brake andthe third and fourth clutches, a forward second speed stage, implementedby operating the first and second brakes, and the fourth clutch, aforward third speed stage, implemented by operating the second brake andthe third and fourth clutches, a forward fourth speed stage, implementedby operating the second brake and the second and fourth clutches, aforward fifth speed stage, implemented by operating the second brake andthe first and fourth clutches, a forward sixth speed stage, implementedby operating the first, second, and fourth clutches, a forward seventhspeed stage, implemented by operating the second brake and the first andsecond clutches, a forward eighth speed stage, implemented by operatingthe second brake and the first and third clutches, a forward ninth speedstage, implemented by operating the first brake and the first and thirdclutches, and a reverse-speed stage, implemented by operating the firstbrake and the second and fourth clutches.

The planetary gear train according to the various embodiments of thepresent invention achieves nine forward speed stages and one reversespeed stage by combining four planetary gear sets with six controlelements.

In addition, the planetary gear train according to various embodimentsof the present invention achieves speed stages suitable to a rotationalspeed of the engine due to multiple-speed stages of the automatictransmission. Particularly, silent driving of the vehicle is improved byusing an operation point positioned at a low rotational speed region ofan engine.

In addition, the planetary gear train according to various embodimentsof the present invention maximizes driving efficiency of the engine andimproves power delivery performance and fuel consumption.

It is understood that the term “vehicle” or “vehicular” or other similarterms as used herein is inclusive of motor vehicles in general such aspassenger automobiles including sports utility vehicles (SUV), buses,trucks, various commercial vehicles, watercraft including a variety ofboats and ships, aircraft, and the like, and includes hybrid vehicles,electric vehicles, plug-in hybrid electric vehicles, hydrogen-poweredvehicles and other alternative fuel vehicles (e.g., fuel derived fromresources other than petroleum). As referred to herein, a hybrid vehicleis a vehicle that has two or more sources of power, for example, bothgasoline-powered and electric-powered vehicles.

The methods and apparatuses of the present invention have other featuresand advantages which will be apparent from or are set forth in moredetail in the accompanying drawings, which are incorporated herein, andthe following Detailed Description, which together serve to explaincertain principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a planetary gear train according tovarious embodiments of the present invention.

FIG. 2 is an operation chart of control elements at each speed stage inthe planetary gear train according to various embodiments of the presentinvention.

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variousfeatures illustrative of the basic principles of the invention. Thespecific design features of the present invention as disclosed herein,including, for example, specific dimensions, orientations, locations,and shapes will be determined in part by the particular intendedapplication and use environment.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of thepresent invention(s), examples of which are illustrated in theaccompanying drawings and described below. While the invention(s) willbe described in conjunction with exemplary embodiments, it will beunderstood that the present description is not intended to limit theinvention(s) to those exemplary embodiments. On the contrary, theinvention(s) is/are intended to cover not only the exemplaryembodiments, but also various alternatives, modifications, equivalentsand other embodiments, which may be included within the spirit and scopeof the invention as defined by the appended claims.

FIG. 1 is a schematic diagram of a planetary gear train according tovarious embodiments of the present invention.

Referring to FIG. 1, a planetary gear train according to variousembodiments of the present invention includes first, second, third, andfourth planetary gear sets PG1, PG2, PG3, and PG4 disposed on the sameaxis, an input shaft IS, an output shaft OS, eight shafts TM1 to TM8connected to at least one of rotation elements of the first, second,third, and fourth planetary gear sets PG1, PG2, PG3, and PG4, fourclutches C1 to C4 and two brakes B1 and B2 that are control elements,and a transmission housing H.

Torque input from the input shaft IS is changed by cooperation of thefirst, second, third, and fourth planetary gear sets PG1, PG2, PG3, andPG4, and the changed torque is output through the output shaft OS.

The planetary gear sets are disposed in the order of first, second,third and fourth planetary gear sets PG1, PG2, PG3 and PG4 from anengine side.

The input shaft IS is an input member and the torque from a crankshaftof an engine, after being torque-converted through a torque converter,is input into the input shaft IS.

The output shaft OS is an output member, is disposed in parallel withthe input shaft IS, and transmits driving torque to a driving wheelthrough a differential apparatus.

The first planetary gear set PG1 is a single pinion planetary gear setand includes a first sun gear 51, a first planet carrier PC1 rotatablysupporting a first pinion P1 that is externally meshed with the firstsun gear S1, and a first ring gear R1 that is internally meshed with thefirst pinion P1 respectively as first, second, and third rotationelements N1, N2, and N3.

The second planetary gear set PG2 is a single pinion planetary gear setand includes a second sun gear S2, a second planet carrier PC2 rotatablysupporting a second pinion P2 that is externally meshed with the secondsun gear S2, and a second ring gear R2 that is internally meshed withthe second pinion P2 respectively as fourth, fifth, and sixth rotationelements N4, N5, and N6.

The third planetary gear set PG3 is a single pinion planetary gear setand includes a third sun gear S3, a third planet carrier PC3 rotatablysupporting a third pinion P3 that is externally meshed with the thirdsun gear S3, and a third ring gear R3 that is internally meshed with thethird pinion P3 respectively as seventh, eighth, and ninth rotationelements N7, N8, and N9.

The fourth planetary gear set PG4 is a single pinion planetary gear setand includes a fourth sun gear S4, a fourth planet carrier PC4 rotatablysupporting a fourth pinion P4 that is externally meshed with the fourthsun gear S4, and a fourth ring gear R4 that is internally meshed withthe fourth pinion P4 respectively as tenth, eleventh, and twelfthrotation elements N10, N11, and N12.

The first rotation element N1 is directly connected to the fifthrotation element N5 and the ninth rotation element N9, the thirdrotation element N3 is directly connected to the seventh rotationelement N7, and the eighth rotation element N8 is directly connected tothe twelfth rotation element N12 such that the first, second, third, andfourth planetary gear sets PG1, PG2, PG3, and PG4 include eight shaftsTM1 to TM8.

The eight shafts TM1 to TM8 will be described in further detail.

The eight shafts TM1 to TM8 directly connect a plurality of rotationelements among the rotation elements of the planetary gear sets PG1,PG2, PG3, and PG4, are rotation members that are each directly connectedto at least one rotation element of the planetary gear sets PG1, PG2,PG3, and PG4 and rotate with the at least one rotation element totransmit torque, or are fixed members that directly connect at least onerotation element of the planetary gear sets PG1, PG2, PG3, and PG4 tothe transmission housing H to fix the at least one rotation element.

The first shaft TM1 directly connects the first rotation element N1(first sun gear S1) to the fifth rotation element N5 (second planetcarrier PC2) and the ninth rotation element N9 (third ring gear R3), andis selectively connectable to the transmission housing H so as to beoperated as a selective fixed element.

The second shaft TM2 is connected to the second rotation element N2(first planet carrier PC1) and is directly connected to the input shaftIS so as to be continuously operated as an input element.

The third shaft TM3 directly connects the third rotation element N3(first ring gear R1) to the seventh rotation element N7 (third sun gearS3).

The fourth shaft TM4 is directly connected to the fourth rotationelement N4 (second sun gear N4) and is selectively connectable to thetransmission housing H so as to be operated as a selective fixedelement.

The fifth shaft TM5 is connected to the sixth rotation element N6(second ring gear R3) and is selectively connectable to the third shaftTM3.

The sixth shaft TM6 directly connects the eighth rotation element N8(third planet carrier PC3) to the twelfth rotation element N12 (fourthring gear R4) and is selectively connectable to the third shaft TM3.

The seventh shaft TM7 is connected to the tenth rotation element N10(fourth sun gear S4) and is selectively connectable to the fourth shaftTM4.

The eighth shaft TM8 is connected to the eleventh rotation element N11(fourth planet carrier PC4), is selectively connectable to the thirdshaft TM3, and is directly connected to the output shaft OS so as to becontinuously operated as an output element.

In addition, four clutches C1, C2, C3, and C4 are disposed at portionsat which any two shafts among the eight shafts TM1 to TM8 including theinput shaft IS and the output shaft OS are selectively connectable toeach other.

In addition, two brakes B1 and B2 are disposed at portions at which atleast one shaft among the eight shafts TM1 to TM8 is selectivelyconnectable to the transmission housing H.

Arrangements of the four clutches C1 to C4 and two brakes B1 and B2 aredescribed in detail.

The first clutch C1 is disposed between the third shaft TM3 and theeighth shaft TM8 and selectively connects the third shaft TM3 to theeighth shaft TM8.

The second clutch C2 is disposed between the third shaft TM3 and thefifth shaft TM5 and selectively connects the third shaft TM3 to thefifth shaft TM5.

The third clutch C3 is disposed between the third shaft TM3 and thesixth shaft TM6 and selectively connects the third shaft TM3 to thesixth shaft TM6.

The fourth clutch C4 is disposed between the fourth shaft TM4 and theseventh shaft TM7 and selectively connects the fourth shaft TM4 to theseventh shaft TM7.

The first brake B1 is disposed between the first shaft TM1 and thetransmission housing H and selectively connects the first shaft TM1 tothe transmission housing H.

The second brake B2 is disposed between the fourth shaft TM4 and thetransmission housing H and selectively connects the fourth shaft TM4 tothe transmission housing H.

The control elements including the first, second, third, and fourthclutches C1, C2, C3, and C4 and the first and second brakes B1 and B2may be multi-plates friction elements of wet type that are operated byhydraulic pressure.

FIG. 2 is an operation chart of control elements at each speed stage inthe planetary gear train according to various embodiments of the presentinvention.

As shown in FIG. 2, three control elements among the first, second,third, and fourth clutches C1, C2, C3, and C4 and the first and secondbrakes B1 and B2 that are control elements are operated at each speedstage in the planetary gear train according to various embodiments ofthe present invention.

The first brake B1 and the third and fourth clutches C3 and C4 areoperated at a first forward speed stage D1.

In a state that the third shaft TM3 is connected to the sixth shaft TM6by operation of the third clutch C3 and the fourth shaft TM4 isconnected to the seventh shaft TM7 by operation of the fourth clutch C4,rotation speed of the input shaft IS is input to the second shaft TM2.In addition, the first shaft TM1 is operated as the fixed element byoperation of the first brake B1. Therefore, the rotation speed of theinput shaft IS is shifted into the first forward speed stage, and thefirst forward speed stage is output through the output shaft OSconnected to the eighth shaft TM8.

The first and second brakes B1 and B2 and the fourth clutch C4 areoperated at a second forward speed stage D2.

In a state that the fourth shaft TM4 is connected to the seventh shaftTM7 by operation of the fourth clutch C4, the rotation speed of theinput shaft IS is input to the second shaft TM2. In addition, the firstshaft TM1 and the fourth shaft TM4 are operated as the fixed elements byoperation of the first and second brakes B1 and B2. Therefore, therotation speed of the input shaft IS is shifted into the second forwardspeed stage, and the second forward speed stage is output through theoutput shaft OS connected to the eighth shaft TM8.

The second brake B2 and the third and fourth clutches C3 and C4 areoperated at a third forward speed stage D3.

In a state that the third shaft TM3 is connected to the sixth shaft TM6by operation of the third clutch C3 and the fourth shaft TM4 isconnected to the seventh shaft TM7 by operation of the fourth clutch C4,the rotation speed of the input shaft IS is input to the second shaftTM2. In addition, the fourth shaft TM4 is operated as the fixed elementby operation of the second brake B2. Therefore, the rotation speed ofthe input shaft IS is shifted into the third forward speed stage, andthe third forward speed stage is output through the output shaft OSconnected to the eighth shaft TM8.

The second brake B2 and the second and fourth clutches C2 and C4 areoperated at a fourth forward speed stage D4.

In a state that the third shaft TM3 is connected to the fifth shaft TM5by operation of the second clutch C2 and the fourth shaft TM4 isconnected to the seventh shaft TM7 by operation of the fourth clutch C4,the rotation speed of the input shaft IS is input to the second shaftTM2. In addition, the fourth shaft TM4 is operated as the fixed elementby operation of the second brake B2. Therefore, the rotation speed ofthe input shaft IS is shifted into the fourth forward speed stage, andthe fourth forward speed stage is output through the output shaft OSconnected to the eighth shaft TM8.

The second brake B2 and the first and fourth clutches C1 and C4 areoperated at a fifth forward speed stage D5.

In a state that the third shaft TM3 is connected to the eighth shaft TM8by operation of the first clutch C1 and the fourth shaft TM4 isconnected to the seventh shaft TM7 by operation of the fourth clutch C4,the rotation speed of the input shaft IS is input to the second shaftTM2. In addition, the fourth shaft TM4 is operated as the fixed elementby operation of the second brake B2. Therefore, the rotation speed ofthe input shaft IS is shifted into the fifth forward speed stage, andthe fifth forward speed stage is output through the output shaft OSconnected to the eighth shaft TM8.

The first, second, and fourth clutches C1, C2, and C4 are operated at asixth forward speed stage D6.

The third shaft TM3 is connected to the eighth shaft TM8 by operation ofthe first clutch C1, the third shaft TM3 is connected to the fifth shaftTM5 by operation of the second clutch C2, and the fourth shaft TM4 isconnected to the seventh shaft TM7 by operation of the fourth clutch C4such that the first, second, third, and fourth planetary gear sets PG1,PG2, PG3, and PG4 become lock-up states. At this state, the rotationspeed of the input shaft IS is input to the second shaft TM2 and thesixth forward speed stage is output through the output shaft OSconnected to the eighth shaft TM8. At the sixth forward speed stage, thesame rotation speed as the input shaft IS is output.

The second brake B2 and the first and second clutches C1 and C2 areoperated at a seventh forward speed stage D7.

In a state that the third shaft TM3 is connected to the eighth shaft TM8by operation of the first clutch C1 and the third shaft TM3 is connectedto the fifth shaft TM5 by operation of the second clutch C2, therotation speed of the input shaft IS is input to the second shaft TM2.In addition, the fourth shaft TM4 is operated as the fixed element byoperation of the second brake B2. Therefore, the rotation speed of theinput shaft IS is shifted into the seventh forward speed stage, and theseventh forward speed stage is output through the output shaft OSconnected to the eighth shaft TM8.

The second brake B2 and the first and third clutches C1 and C3 areoperated at an eighth forward speed stage D8.

In a state that the third shaft TM3 is connected to the eighth shaft TM8by operation of the first clutch C1 and the third shaft TM3 is connectedto the sixth shaft TM6 by operation of the third clutch C3, the rotationspeed of the input shaft IS is input to the second shaft TM2. Inaddition, the fourth shaft TM4 is operated as the fixed element byoperation of the second brake B2. Therefore, the rotation speed of theinput shaft IS is shifted into the eighth forward speed stage, and theeighth forward speed stage is output through the output shaft OSconnected to the eighth shaft TM8.

The first brake B1 and the first and third clutches C1 and C3 areoperated at a ninth forward speed stage D9.

In a state that the third shaft TM3 is connected to the eighth shaft TM8by operation of the first clutch C1 and the third shaft TM3 is connectedto the sixth shaft TM6 by operation of the third clutch C3, the rotationspeed of the input shaft IS is input to the second shaft TM2. Inaddition, the first shaft TM1 is operated as the fixed element byoperation of the first brake B1. Therefore, the rotation speed of theinput shaft IS is shifted into the ninth forward speed stage, and theninth forward speed stage is output through the output shaft OSconnected to the eighth shaft TM8.

The first brake B1 and the second and fourth clutches C2 and C4 areoperated at a reverse speed stage REV.

In a state that the third shaft TM3 is connected to the fifth shaft TM5by operation of the second clutch C2 and the fourth shaft TM4 isconnected to the seventh shaft TM7 by operation of the fourth clutch C4,the rotation speed of the input shaft IS is input to the second shaftTM2. In addition, the first shaft TM1 is operated as the fixed elementby operation of the first brake B1. Therefore, the rotation speed of theinput shaft IS is shifted into the reverse speed stage, and the reversespeed stage is output as an inverse rotation speed through the outputshaft OS connected to the eighth shaft TM8.

The planetary gear trains according to various embodiments of thepresent invention achieves at least nine forward speed stages and onereverse speed stage by combining four planetary gear sets PG1, PG2, PG3,and PG4 with the four clutches C1, C2, C3, and C4 and the two brakes B1and B2.

In addition, the planetary gear train according various embodiments ofthe present invention achieves suitable speed stages according torotation speed of the engine. Particularly, silent driving of thevehicle may be improved by using operation point positioned at a lowrotational speed region of the engine.

Furthermore, the planetary gear train according to various embodimentsof the present invention maximizes driving efficiency of the engine andimproves power delivery performance and fuel consumption.

The foregoing descriptions of specific exemplary embodiments of thepresent invention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteachings. The exemplary embodiments were chosen and described in orderto explain certain principles of the invention and their practicalapplication, to thereby enable others skilled in the art to make andutilize various exemplary embodiments of the present invention, as wellas various alternatives and modifications thereof. It is intended thatthe scope of the invention be defined by the Claims appended hereto andtheir equivalents.

What is claimed is:
 1. A planetary gear train of an automatictransmission for a vehicle comprising: an input shaft receiving torqueof an engine; an output shaft outputting torque of the engine; a firstplanetary gear set including a first rotation element, a second rotationelement, and a third rotation element; a second planetary gear setincluding a fourth rotation element, a fifth rotation element, and asixth rotation element; a third planetary gear set including a seventhrotation element, an eighth rotation element, and a ninth rotationelement; a fourth planetary gear set including a tenth rotation element,an eleventh rotation element, and a twelfth rotation element; a firstshaft connecting the first rotation element to the fifth rotationelement and the ninth rotation element; a second shaft connected to thesecond rotation element and directly connected to the input shaft; athird shaft connecting the third rotation element to the seventhrotation element; a fourth shaft connected to the fourth rotationelement; a fifth shaft connected to the sixth rotation element andselectively connectable to the third shaft; a sixth shaft connecting theeighth rotation element to the twelfth rotation element and selectivelyconnectable to the third shaft; a seventh shaft connected to the tenthrotation element and selectively connectable to the fourth shaft; and aneighth shaft connected to the eleventh rotation element, selectivelyconnectable to the third shaft, and directly connected to the outputshaft.
 2. The planetary gear train of claim 1, wherein the first shaftand the fourth shaft are each selectively connectable to a transmissionhousing.
 3. The planetary gear train of claim 1, wherein the firstrotation element, the second rotation element, and the third rotationelement comprise a first sun gear, a first planet carrier, and a firstring gear, the fourth rotation element, the fifth rotation element, andthe sixth rotation element comprise a second sun gear, a second planetcarrier, and a second ring gear, the seventh rotation element, theeighth rotation element, and the ninth rotation element comprise a thirdsun gear, a third planet carrier, and a third ring gear, and the tenthrotation element, the eleventh rotation element, and the twelfthrotation element comprise a fourth sun gear, a fourth planet carrier,and a fourth ring gear.
 4. The planetary gear train of claim 2, furthercomprising: a first clutch selectively connecting the third shaft to theeighth shaft; a second clutch selectively connecting the third shaft tothe fifth shaft; a third clutch selectively connecting the third shaftto the sixth shaft; a fourth clutch selectively connecting the fourthshaft to the seventh shaft; a first brake selectively connecting thefirst shaft to the transmission housing; and a second brake selectivelyconnecting the fourth shaft to the transmission housing.
 5. A planetarygear train of an automatic transmission for a vehicle comprising: aninput shaft receiving torque of an engine; an output shaft outputtingtorque; a first planetary gear set including a first rotation element, asecond rotation element, and a third rotation element; a secondplanetary gear set including a fourth rotation element, a fifth rotationelement, and a sixth rotation element; a third planetary gear setincluding a seventh rotation element, an eighth rotation element, and aninth rotation element; and a fourth planetary gear set including atenth rotation element, an eleventh rotation element, and a twelfthrotation element, wherein the input shaft is directly connected to thesecond rotation element, the output shaft is directly connected to theeleventh rotation element, the first rotation element is directlyconnected to the fifth rotation element and the ninth rotation element,the third rotation element is directly connected to the seventh rotationelement, the sixth rotation element is selectively connectable to thethird rotation element, the eighth rotation element is directlyconnected to the twelfth rotation element and selectively connectable tothe seventh rotation element, the tenth rotation element is selectivelyconnectable to the fourth rotation element, and the eleventh rotationelement is selectively connectable to the third rotation element.
 6. Theplanetary gear train of claim 5, wherein the first rotation element andthe fourth rotation element are each selectively connectable to atransmission housing.
 7. The planetary gear train of claim 5, whereinthe first rotation element, the second rotation element, and the thirdrotation element comprise a first sun gear, a first planet carrier, anda first ring gear, the fourth rotation element, the fifth rotationelement, and the sixth rotation element comprise a second sun gear, asecond planet carrier, and a second ring gear, the seventh rotationelement, the eighth rotation element, and the ninth rotation elementcomprise a third sun gear, a third planet carrier, and a third ringgear, and the tenth rotation element, the eleventh rotation element, andthe twelfth rotation element comprise a fourth sun gear, a fourth planetcarrier, and a fourth ring gear.
 8. The planetary gear train of claim 6,further comprising: a first clutch selectively connecting the seventhrotation element to the eleventh rotation element; a second clutchselectively connecting the third rotation element to the sixth rotationelement; a third clutch selectively connecting the seventh rotationelement to the eighth rotation element; a fourth clutch selectivelyconnecting the fourth rotation element to the tenth rotation element; afirst brake selectively connecting the first rotation element to thetransmission housing; and a second brake selectively connecting thefourth rotation element to the transmission housing.
 9. The planetarygear train of claim 8, wherein speed stages implemented by selectivelyoperating the first, second, third and fourth clutches and the first andsecond brakes include: a first forward speed stage, implemented byoperating the first brake and the third and fourth clutches; a secondforward speed stage, implemented by operating the first and secondbrakes, and the fourth clutch; a third forward speed stage, implementedby operating the second brake and the third and fourth clutches; afourth forward speed stage, implemented by operating the second brakeand the second and fourth clutches; a fifth forward speed stage,implemented by operating the second brake and the first and fourthclutches; a sixth forward speed stage, implemented by operating thefirst, second, and fourth clutches; a seventh forward speed stage,implemented by operating the second brake and the first and secondclutches; an eighth forward speed stage, implemented by operating thesecond brake and the first and third clutches; a ninth forward speedstage, implemented by operating the first brake and the first and thirdclutches; and a reverse-speed stage, implemented by operating the firstbrake and the second and fourth clutches.